Mercruiser 3.0: Dead cylinder? very sluggish!

[Chaparral260]

Had the boat out on the water for 4 or 5 days. Pulled a water skier then went for a boat ride, floated for a while then head back to the dock and would barely get on a plane. Motor sounded different, Motor sounds like it’s bogged down.

Motor starts normal. Sounds different when running. Low rpm seems normal, when hit the gas or speed up it is very lugging, barely get on plane.

Steps I have taken to diagnose and troubleshoot the problem:
- replaced spark plugs
- replaced distributor cap
- replaced spark plug wires
- replaced fuel filter
- compression test (175, 168, 165, 160)
- replaced gas with fresh gas
- running with spark arrester off, squirted gas into carburetor. Boat didn’t speed up, bogged down a little more.
- pulled spark plug wire of one spark plug at a time. Then started and try to get boat on plane. Cylinder 2,34 all had hard time starting and could not get on plane. Cylinder 1 disconnected ran the same when connected. (Counting cylinder from stern to bow).
- checked spark plug again. #1 plug was black powder resid. Replaced with another spark plug.

1999 mercruiser 3.0L
SN: L339355
Spark plug: NKG BPR6EFS

I am at wits end! What else could it be? What else can I try?

 

[nola mike]

Sounds like you've got a dead #1 cylinder. I'd start by throwing a timing light on #1, and/or a spark plug tester

 

[Bondo]

(Counting cylinder from stern to bow).

Ayuh,.... Welcome Aboard,..... #1 is closest to the bow, #4 is nearer the stern,....

 

[kenny nunez]

You could be having water incursion from the exhaust manifold riser In the rear #4 cylinder. Connect a garden hose to the front of the manifold where the thermostat housing discharges. With the plugs out spin the engine and look if droplets of water appear.

 

[Rick Stephens]

I'd consider pulling the valve cover off and making sure rockers are moving same on #1 as others.

 

[Chaparral260]

Thanks for the suggestions

here is my update
Update:
- spark tester flashed same on all 4 at idle and slow throttle up.
- timing was close to 1 ATDC
- put garden hose on hose that comes out of thermostat housing and goes to manifold. I let water run fir 5 min then cranked over a few times. Let it start once . No drop lets that I could see. Not sure how obvious it would be.

wondering if it is a stuck valve? Will pull the cover off next, but won’t be for a few days.

 

[Chaparral260]

Sooo (skip to valve - for attempts that didn’t help)
First did a sea foam clean. Warmed engine up, Pulled spark plug, sprayed in cylinders, sat for an hour, started & idles, then ran fat idle & sprayed in carb. Repeat 3 times. Hoping stuck valve would free.
Second, did a oil flush. Napa brand. Heat engine, remove quart to make room for flush, add oil flush, idle for 6 minutes, change oil & filter.
Test drive. Still lugging but engine runs good. Like a really good 3 cylinder, . Still just above plane at wide open.
Valve cover,
Pulled valve cover. On 4th cylinder the last valve (to stern) would lift. The next valve would not move. Took both rocker off and pulled push rod on 4th. Push rod looked same, could see rub point but no wear. Put rod back in (left rocker off). Turned engine. Last rod would go up, next rod would not. Clearly the rod isn’t pushing the rocker arm and thus not opening the valve.

So what does this mean? Can?

is it ok if I use it with only 3 cylinder with out doing mor damage.

thanks again for everyone help.

 

[kenny nunez]

You probably have a worn out cam lobe. The lubricants today are not compatible with a flat tappet store cam. You have to either obtain oil with the additives or add the additives lead substitute to the oil. Sierra Marine has the oil with the additives and you can also purchase the oil from any high performance engine builder.
And it probably can be obtained at some large chain stores.
What you want is an oil that has ZDDP additives.
But for the time being you will have to pull the engine to clean out the camshaft chips and replace the camshaft and lifters.

 

[Chaparral260]

Thanks for the info - I wish I had the time to take on a rebuild but likely have to go to the shop.

 

[Scott Danforth]

Thanks for the info - I wish I had the time to take on a rebuild but likely have to go to the shop.

for a cam change?

pull motor, put on stand, pull pan
hose out crank case with brake clean, removing the bits of trash
pull cam and lifters and throw away
install new cam and lifters
install pan
fill with 15W40 diesel oil (it still has zinc) and add extra zinc for break-in
re-install motor
align motor
back boat into water at ramp on a quiet day
run motor at 2000 RPM for 20 mintues to break in cam and lifters
go boating